A carbon nanotube is a substance having a structure in which a graphite sheet composed of a hexagonal mesh-shaped arrangement of carbon atoms is cylindrically wound, and a carbon nanotube of which the graphite sheet is wound into a single layer is called a single-walled carbon nanotube, and that of which the graphite sheet is wound into multiple layers is called a multi-walled carbon nanotube. Among the multi-walled carbon nanotubes, in particular, a carbon nanotube of which the graphite sheet is wound into two layers is called a double-walled carbon nanotube.
Known methods of producing carbon nanotubes include synthesis methods by a laser ablation method, a chemical vapor deposition method (a CVD method) and the like. Among the above, the CVD method is a synthesis method capable of controlling reaction conditions such as the type of the carbon starting material, supply rate of the starting material, synthesis temperature, and density of the catalyst, and is capable of synthesizing carbon nanotubes in a large scale relatively easily. Recently, it is becoming possible to selectively synthesize a carbon nanotube having desired diameter, length and number of layers, and it is known that a chemical vapor deposition method is capable of producing a carbon nanotube by controlling the number of layers of carbon nanotubes to a single layer or two to five layers.
Carbon nanotubes are expected to be used as electrically conductive materials because they themselves have excellent electrical conductivity. It is known that, normally, among carbon nanotubes, those having a small number of layers such as a single-walled carbon nanotube and a double-walled carbon nanotube have especially excellent properties such as electrical conductivity because they have a high rate of the graphite structure.
Carbon nanotubes have been used for a transparent conductive film and the like by taking advantage of these properties. In addition, the transparent conductive film can be suitably used as a substrate having a transparent electrically conductive film such as a touch panel, a liquid crystal display, organic electroluminescence, and electronic paper. In these uses, a transparent conductive film having higher electrical conductivity is required, and an aggregate of carbon nanotubes capable of exhibiting excellent electrical conductivity when used to configure a transparent conductive film is desired.
To obtain an aggregate of carbon nanotubes capable of exhibiting excellent electrical conductivity when used to configure a transparent conductive film, methods of removing carbon impurities other than carbon nanotubes contained as impurities in a composition containing carbon nanotubes produced by a CVD method such as amorphous carbon and particulate carbon, have been known. As a method of removing carbon impurities other than carbon nanotubes, a heating method in a gaseous phase is generally used. However, when a composition containing carbon nanotubes is heated in a gaseous phase, not only carbon impurities are removed, but also defects occur in the outer layer of a multi-walled carbon nanotube at the same time. Accordingly, a technique of removing the outer layer having defects by performing a liquid phase oxidation treatment on a composition containing carbon nanotubes which has been subjected to vapor phase oxidation (Japanese Patent Laid-open Publication No. 2009-012988) has been known, but the electrical conductivity of the resulting aggregate of carbon nanotubes is insufficient. In addition, when the liquid phase oxidation treatment is performed, use of a stronger acid tends to make removal of carbon impurities easier, but use of a strong acid tends to damage carbon nanotubes, which results in impaired properties of the carbon nanotubes. Therefore, it is practically required to use an acid with relatively mild reactivity as an acid to remove carbon impurities.
Further, except for removal of carbon impurities, as a method of improving electrical conductivity of an aggregate of carbon nanotubes, a method of adding a dopant has been proposed. So far, various compounds have been studied as dopants, and an example of adding thionyl chloride as a dopant has been reported, but sufficient electrical conductivity has not been achieved and, further, thionyl chloride has a problem with regard to stability as a dopant (Japanese Patent Laid-open Publication No. 2009-536911).
As methods of improving electrical conductivity other than those described above, a method of separating metallic carbon nanotubes having excellent electrical conductivity from semiconductive carbon nanotubes by electrophoresis, a synthesis method which employs metallic nanotubes as a main component in a synthesis stage and the like have been proposed, but these methods are poor in reproducibility because their operations are very complex and, therefore, they have not been put to practical use.
It could therefore be helpful to provide an aggregate of carbon nanotubes capable of exhibiting excellent electrical conductivity when used to configure a transparent conductive film, and a production method therefor.